Sedimentary Characters and Environmental Magnetism Analses of Two Cores in Kunming Basin

Abstract: Kunming Basin is a late Cenozoic fault lake basin in Yunnan Plateau with an area of aboui 1. 000km². The present lake Dianchi is about 300km~3 in area with its mean water level 1,886m a.s.l.The two cores (Chad Chao2) taken from the east shore of Chaohai Bay of the northern Dianchi Lake were ac-cumululed in the latest 50, 000 years with a depth of about 20m of each. The sites where the two cores are drilled was in the environment with lower energy all the time, so he sediments are ralatively fine, most of their mean grain sizes are above 4φ. Based on the analyses of lithological characters, sedimentary structures, grain sizes and biological fossils, four main sedimentary facieses may be identified in the two cores, i.e, deltaic plain, lacus, delta-front and lakeside swamp, adn may be subdivided into nine subfacieses. The average sedimentation rates of the two cores are about 40cm/1000a. Magnetite with a little maghemite and haematite are the main ferromagnetic minerals in the sediments, which are concentrated in teh silt fraction, especially in teh fine and very fine fractions. The secondary magnetite derived from the catchment may cling to coarser clay partiche's as a pellicle.The sediment susceptibility value is related with the sediment grain size composition. The susceptibility measurement made on the cores' sediments show that in deltaic plain sediments the susceptibility values increase as the sedimentary dynamic decreases, the susceptibility values of distributary, natural level and flood plain sediments are 31.7× 10^-8, 62.4× 10^-8, and 154.1× 10^-8SI, respectively.The susceptibility value of subaqueous river sediment is 46.3 × 10^-8SI, that of river mouth bar sediment is 71.4× 10^-8SI, lake-shore sediment 34× 10^-8SI, off-shore shallow lake sediment 129. 7× 10^-8SI.The lowest susceptibility value of swamp sediment is only 3.8 × 10^-8SI.The sediment related with human activity has a very high susceptibility value of 260.7 × 10^-8SI. The two cores' susceptibility curves are characterized by Ⅰ. intermediate variations and comparatively higher susceptibility values of deltaic plain sedimemts; Ⅱ. great variations and high susceptibility values of lake sediments, reflection the transgression adn regression of the lake; Ⅲ .smaller variations and comparatively lower susceptibility values of lake-shore and delta-front sediments; Ⅳ.comparatively stable and lower susceptibility values of lake-side swamp sediment; Ⅴ .very high susceptibility values of the sediment related with human activity. Both of the two cores show a complete lake transgression.It mainly occured between 38, 000 and 28, 000 years B.P., and may be correlated to the third oxygen-isotope stage recorded in deep sea cores.The susceptibility curves drawed according to the results of susceptibility measursment may give evidences to the transgression and regression of the lake and the sedimentary environmental changes.